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A contaminated Welsh site needed a good scrub-up before its transformation into a sports and leisure village could begin.

Hugh Cherrill reports.

Cardiff International Sports Village is a £700M redevelopment project in Cardiff Bay. The 35ha former harbour and industrial site will be transformed into a mix of sports, leisure, commercial and residential uses.

These include ski slopes, an ice hockey stadium, golf, cricket, rugby, football and water sport facilities.

It will also have a cinema, 1000 flats, art galleries and a 26-storey residential tower.

Major remediation contracts and river edge works on the land owned by Cardiff County Council were recently completed. Infrastructure construction, including access roads, is now under way and contractors will soon begin building the swimming pool complex and a retail area.

The site has a long industrial history. Originally mudflats, with a meander of the River Ely crossing it, it was developed in the latter half of the 19th century as a tidal harbour for coal export.

Reclamation works at that time involved construction of a revetted bund around the river and bay edges, with infilling behind the bund to raise the ground level.

By 1941 an oil terminal occupied a large part of the site. In the 1970s the alignment of the River Ely was straightened to cut off the meander, which was then used as a municipal refuse tip. The site has also been used for a variety of other contaminating activities such as scrap yards and a waste transfer station.

Arup won the contract to assess the extent of contamination and need for remedial action using a desk study, site investigations and risk assessments.

The desk study, which began in 1998, identified a wide range of potential contamination, mainly heavy metal and hydrocarbons, as well as pollutants associated with the landfill area.

Two phases of site investigation followed to determine the nature and extent of contamination and its significance in relation to groundwater, surface water and the proposed end uses.

Investigations identified a large area of hydrocarbon contamination (mainly diesel fuel) in the area of the former oil terminal with other smaller, but significant hydrocarbon contaminated areas associated with other previous uses, such as the scrap yard. Arup documented extensive heavy metal contamination and, as expected, found landfill leachate and gas in the old refuse tip.

Assessment of the data indicated a number of unacceptable risks:

hydrocarbon and heavy metal contamination present at levels unacceptable for human health, particularly in housing areas;

hydrocarbon contamination of soil and groundwater at levels presenting a significant risk to the water quality of Cardiff Bay;

leachate in the refuse tip presenting a significant risk to the water quality of Cardiff Bay; and

landfill gas generation in the tip presenting a potentially unacceptable risk to the development.

Arup developed a remediation strategy, agreed with Cardiff County Council and the Environment Agency and incorporated in a £15M design and build contract. This was let to Taylor Woodrow Construction with Arup acting as consultant on remediation issues.

Subcontractor Churngold Remediation used exsitu biopiles for 28,000m3 of hydrocarbon contaminated soils, and insitu bioremediation for 17,000m3 of soil and groundwater using Isogen electrolysis cells in wells.

Other work saw removal of hydrocarbon-free product from the water table and the installation of a 700m long cement bentonite slurry cut-off wall, incorporating an HDPE membrane, along the edge of the refuse tip to control gas and leachate migration.

Site workers installed a drainage system and the refuse tip needed capping and gas control measures.

On top of all this, 26,000m3 of soil contaminated with Japanese knotweed needed treatment.

These measures were combined with other redevelopment work that included dynamic and vibrocompaction of the 95,000m2 refuse tip, construction of a 1600m long sheet pile retaining wall to the river and bay edge and 300,000m3 of earthworks.

The only materials removed from site were small quantities of rubbish such as tyres and refuse and a small amount of asbestos contaminated soil, while 50,000m3 of concrete was crushed and reused in the works. All other material remained onsite after treatment or by risk classification and use in areas of appropriate end-use.

This approach contributed substantially to the economy and sustainability of the project that obtained an 'excellent' Ceequal (civil engineering environmental quality assessment and award scheme) rating of 91.9%, the second highest achieved to date.

The remediation strategy was based on the requirement that developers would take further measures to protect human health.

These include covering soil with buildings, hard standing or 600mm of clean soil and providing gas control measures. The latter is particularly important for development of the refuse tip area where roads, car parks and large retail units are planned.

Taylor Woodrow completed the remediation works last year, allowing Laing O'Rourke to begin the £15M infrastructure contract it won from Cardiff Bay Infrastructure.

The contractor is employing Arup as designer. Its work includes construction of roads across the landfill, where one challenge is building these on new embankments to join the existing spur road to nearby Cogan.

This road was built over the landfill, about 5m of domestic refuse, in the mid-1980s. Most of the refuse beneath the road was removed and PFA used to form the embankment.

This reduced embankment loading and subsequent settlements arising from consolidation of the soft alluvial clays beneath - typically about 10m thick - throughout the site.

Arup is also focusing on settlement for new road embankments needed to tie in with the existing road.

Settlement is caused by refuse degradation and consolidation of the alluvial clays beneath. The refuse degradation settlement has been assessed based on the improved characteristics of the material after dynamic compaction.

This relies on examination of similar sites, where the amount of degradation settlement over 10 to 20 years could be estimated. For the thickness of refuse, the estimated degradation settlement is 80mm after 50 years.

This settlement is in addition to consolidation of the underlying soft alluvial clays, and to limit total settlements for the road, lightweightfill is playing a part.

But the absolute magnitude of settlement is not the only concern as there are predicted differential settlements that could occur from variation in ground conditions and in embankment height.

To limit the embankment's imposed load, as well as using lightweight fill within the new embankment above existing ground, the contractor will excavate below existing ground and replace with lightweight fill. The material is clay, expanded in rotary kilns at high temperatures to form small 10mm to 20mm lightweight granules with a honeycomb core.

The lightweight fill embankment's cross section varies along its length to provide a pattern of imposed loadings. These will mean acceptable settlement for both total and differential values.

Maximum settlement will be limited to about 150mm to 200mm, with differential settlement to gradients of 1 in 150 or less, for the long term design life of the road.

These criteria were developed to allow for drainage runs within the road's structure that take account of the particular settlements.

Measures have been taken to deal with ground gases in the underlying landfill where the road crosses it.

After borehole monitoring it was recommended that gas venting be incorporated over the full extent of the road construction where it runs over the refuse. The venting should prevent potential gas pressure buildup beneath the road, given that the pavement effectively forms an impermeable surface.

This pressure build-up could lead to localised uncontrolled gas releases at ground level, which might cause odour nuisance, or be more hazardous. Arup proposed a geocomposite venting layer under the road, formed of a cuspated HDPE sheet with geotextile cover.

This should direct gas collected beneath the road to the perimeter verges, for venting to atmosphere.

Lightweight fill work is now under way, and all construction on the International Sports Village is due to complete in 2010.

Hugh Cherrill is an associate at Arup.

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